Hiiragi Yatsuse is a shy and reserved first-year high school student from Yamagata Prefecture, intensely afraid of rejection or disapproval. This fear renders him unable to refuse requests or voice his own needs, leading classmates to exploit him with burdensome tasks like completing their homework, covering chores, or participating in deceptive arrangements such as pretending to be a boyfriend. These interactions consistently disadvantage him, breeding resentment and suppressed emotions.

His life shifts dramatically during an unexpected summer encounter with Tsumugi, a determined girl searching for her mother. Their extensive journey over nearly three-and-a-half days forces repeated confrontations with supernatural snow gods. Throughout this time, Hiiragi's suppressed feelings—particularly frustrations from Tsumugi's assertiveness and unresolved family tensions like avoiding a confrontation about private lessons replacing cram school—physically manifest as small, jellyfish-like entities called "mini-oni" emerging from his body.

Intense emotional distress following a heated separation from Tsumugi triggers his complete physical transformation into an oni. This metamorphosis results in capture by a snow god and transportation to a hidden village. Rescued and welcomed by the villagers, he later shows significant growth by openly confessing his affection for Tsumugi and racing across hazardous terrain to reach her during a critical confrontation. These actions coincide with his reversion to human form.

Returning home, Hiiragi achieves reconciliation within his family and successfully establishes genuine friendships. A subsequent visit from Tsumugi confirms their sustained connection and mutual romantic feelings, though his human status prevents access to her village.